1. Field of the Invention
This invention relates to gettering semiconductor wafers by directing a high energy beam on the wafers, and more particularly to gettering a silicon wafer by inducing lattice damage with a laser directed on, and absorbed by, a surface of the wafer.
2. Description of Prior Art
In the manufacture of semiconductor devices, such as transistors, integrated circuits or the like, semiconductor wafers are separated into the devices by scribing and breaking, sawing, etching or the like. From an economic standpoint, it is of the utmost importance to obtain the highest possible yield of good devices from each wafer. High yields obviously increase the productivity of workers and capital, decrease the unit cost of each device and avoid expenditures on costly capital equipment.
To improve device yields, a number of different gettering techniques have been used in the prior art. These techniques have been effective in reducing various mobile defects in the crystalline structure of wafers from which semiconductor devices are fabricated. Such reduction of these defects, which might include point, line, area, and volume defects, takes place at active or critical portions of the wafers where device processing will occur.
For example, area defects such as stacking faults have been eliminated in silicon devices by a gettering process that includes forming on the back surface of the wafer a stressed layer and then annealing the wafer for a time and temperature sufficient to diffuse nucleation sites to a region near the back surface. The stressed layer may be formed by depositing silicon nitride or aluminum oxide on the back surface of the wafer. Enhanced gettering is achieved, if prior to forming the stressed layer, phosphorus is diffused into the back surface. See U.S. Patent 3,997,368 to Petroff et al. assigned to Bell Laboratories.
Another gettering technique of the prior art is inducing lattice damage by grinding, lapping, grit blasting or incomplete removal of damage resulting from sawing a crystalline ingot into wafers. By so inducing lattice damage, point defects, such as metallic contaminants or out-of-place atoms, are reduced in the regions where devices are to be formed. In this technique, which also includes a heat treatment step, mobile defects migrate and are attached to dislocations resulting from the lattice damage.
Although the prior art technique of inducing lattice damage by grinding, lapping or grit blasting does improve yields, the abrasives used to apply the damage may actually contaminate the wafer. And the amount and uniformity of the damage is uncertain. Also, practical considerations usually limit this gettering technique to pre process use.
Still other gettering techniques of the prior art are ion implantation damage and oxygen precipitates intrinsic to the starting wafer.
These prior art gettering techniques do not have universal application. Some must be applied pre, mid or post process for maximum results, depending on the starting material and the particular processing technique. Also, some of these gettering techniques are incompatible with the particular processing technique or are simply too costly to use.
For example, the prior art lattice damage technique is not easily applied mid process because of the necessity of protecting the side of the wafer where semiconductor devices are to be formed.
When lattice damage is applied pre process in a bipolar process its effectiveness may be reduced by the high temperature furnace operations preceding critical base and emitter diffusions. This is particularly troublesome because gettering is most needed prior to these diffusions to reduce certain defects in the wafers and improve junction yields.
Gettering with pre process phosphorus diffusions is not compatible with later process steps such as epitaxy. This is because such phosphorus may be incorporated into the growing of epitaxial layers by autodoping. Also, such gettering usually requires masking or other protective measures to prevent such phosphorus from contaminating the active side of the wafer where devices are to be formed.
It is desirable to carry out gettering at any point in the process of making semiconductor devices. It is also desirable that such gettering not introduce contamination into the device, as often happens with some prior art techniques.
Additionally, precise control of the gettering (i.e., damage) is desirable so that it may be tailored to any process for fabricating semiconductor devices from wafers. And such control is of great importance in preventing wafer breakage during gettering. Also, it is desirable to be able to readily refresh gettering by an additional application of damage at any point in the process of fabricating the wafer into semiconductor devices.